This is a great core workout I stole from Gym Jones. I enjoy doing this one occasionally because its challenging in two ways. You are working your core for time, so its an endurance exercise. Also increasing the weight of the dumbbell forces your core to work harder so its also a maximal strength exercise too. You can really take this one to the limits.
Use two dumbbells no less than 3/4 of your body weight to secure your feet. Site on the bench and ready your dumbbell on your lap. Lye back and use your core to establish a horizontal form. Start pressing. Try to keep the position when you switch arms, it keeps your core under stress and works it harder. Repeat it but in revers to ensure an even workout (i.e. Start with right then left, next left then right etc). If you are really brave, take on two dumbbells at once, ouch!